[QGIS-Developer] QGIS 3.44 projects compatibility with previous versions

[Denis Rouzaud]

Hi,

I did something similar in the symbology area and I went for saving twice
the data.
We could probably leave the two at the same time in the same tag:

<properties>
  <foo>
    <bar type="QString">baz</bar>
  </foo>
  <properties name="foo">
    <properties name="bar" type="QString">baz</properties>
  </properties>
</properties>

Old QGIS would only save the old, new QGIS would save both and use the new
if it exists.

Best of the 2 worlds?

Cheers,
Denis



Le jeu. 12 juin 2025 à 13:55, Stefanos Natsis via QGIS-Developer <
qgis-developer at lists.osgeo.org> a écrit :

> Hi list
>
> During the last dev cycle, a change was introduced in the way that
> properties are stored in the .qgs xml file
> https://github.com/qgis/QGIS/pull/60855
>
> While this solved a real issue (data-driven properties could result in an
> invalid property name with a leading digit) in an elegant way and is
> technically backwards compatible (projects generated with older versions
> can be opened without issues), it has the side effect that projects stored
> with 3.44 will not properly load in older QGIS versions (stored properties
> are ignored and also project CRS is undefined). Plugins could also
> malfunction as they often use project properties for storing their
> project-related configurations.
>
> One potential solution would be to backport this to LTR, however from my
> experience a good percentage of the users use older QGIS versions and even
> those on LTR do not always upgrade as soon as a new release is out, so
> backporting could eventually make things worse!
>
> With QGIS 4.0 around the corner, imho, it would be better to defer this
> change for a couple more months and only include this kind-of-breaking
> change in QGIS 4.0
>
> I briefly chatted with Jurgen at the hackfest about this and while
> acknowledging the issue, was not super happy to revert, so I'm interested
> in gathering more opinions here.
>
> Best
> Stefanos
>
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